Method for brazing super alloys and refractory metals



P 7 w. T. KAARLELA 3,342,971

METHOD FOR B RAZING SUPER ALLOYS AND REFRACTORY METALS Original FiledJune 16, 1964 2 Sheets-Sheet 1 WILLIAM T. KAARLELA INVENTOR.

ATTORNEY Sept. 19, 1967 w. T. KAARLELA 3,342,971

METHOD FOR BRAZING SUPER ALLOYS AND REFRACTORY METALS Original FiledJune 16, 1964 2 Sheets-Sheet 2 WILLIAM T. KAARLELA INVENTOR.

ZMMM ATTORNEY United States v Patent 3,342,971 METHOD FOR BRAZING SUPERALLOYS AND REFRACTORY METALS William T. Kaarlela, Fort Worth, Tex.,assignor to General Dynamics Corporation, a corporation of DelawareOriginal application June 16, 1964, Ser. No. 375,514. Divided and thisapplication Sept. 7, 1966, Ser. No. 591,071

3 Claims. (Cl. 219-85) ABSTRACT OF THE DISCLOSURE A process for joiningsuper alloy and refractory metals by brazing, using suitable brazematerials and particularly prealloyed powder material mixed with a resinin Water and brushed on the surfaces of the materials to be joined,wherein the materials are positioned within an enclosed environment,purging, slowly increasing the temperature of the materials to be joinedto about 1600" F. and holding at this temperature to permit removal ofthe acrylic resin matrix in the braze alloy, resuming the heatingoperation at a rate of about 200 F. per minute to a temperature justbelow that at which recrystallization begins in the refractory metalswhich are being joined, rapidly raising the mate-rials to the brazingtemperature and rapidly terminating and cooling the thus brazedmaterials to a temperature below that at which recrystallization takesplace, the time interval of the last two operations, i.e., the brazingand cooling, being one minute or less, so that the material is withinits recrystallization zone for no more than sixty seconds and will thusresult only in a maximum of about 5% recrystallization to prevent asignificant effect on the properties of the base materials.

The present invention, a division of my co-pending application, SerialNo. 375,514 filed June 16, 1964, relates generally to metal joining.

More particularly, the invention pertains to a metal joining process forrefractory metals and super alloys.

The present invention is characterized by extremely brief heating andcooling cycles wherein metal joining techniques requiring varioustemperature-pressure relationships may be performed upon materials suchas refractory metals, super alloys and the like, while at the same timeproviding such materials with a controlled environment, thus permittingjoining of these materials to be effected without detrimentalmetallurgical changes occurring therein.

The aerospace industry, in particular, is confronted with the need forstructural materials comprised of refractory metals and super alloys foruse in present and future vehicles. This is due primarily to theinherent strength and stability at ultra-high temperatures of suchmetals. With the growing utilization of refractory metals acorresponding problem has arisen in the need for efficient processing ofcomponents fabricated therefrom. This problem is due to phenomenaassociated with refractory metals which causes them to recrystallizewhen subjected to the temperatures necessary to effect joining orannealmg.

This recrystallization and its associated grain growth permanently andadversely affect the mechanical properties of refractory metal.Therefore, in the processing of such materials prime consideration mustbe given the relationships of temperatures and dwell times to which thematerial must be subjected in order to effect joining. Theserelationships (time and temperature) are critical sincerecrystallization and grain growth begin to occur in the object materialimmediately upon its entering the recrystallization zone. The degree ofrecrystallization is directly proportional to dwell time and to theamount by which the initial recrystallization temperature is exceeded,and thus the total temperate gradient experienced.

To the present time the radiant, vacuum-type furnace has usually beenemployed to join high temperature resistant super alloy or refractorytype metal members.

Inasmuch as this type furnace effectively produces only radiant heat, ithas an exceedingly poor rate of heat transfer. This is due to therelatively wide spacial disposition of the tantalum heaters usedtherein, in relation to the object workpiece as well as to a lack oftransmitting atmosphere. Such furnaces therefore have not been capableof satisfactorily brazing refractory metals and super alloy metals.Because of the exceptionally high current required, the state-of-the-artheaters employed cannot be closely spaced without engendering directarcing or shorting between heater and workpiece.

In the operation of such a conventional radiant heat vacuum furnace, thesupport fixture and insulation masses must also be substantiallysaturated with heat along with the mass of the workpiece because of theabove mentioned widely spaced relationship of heaters and work.Obviously, the aggregate of masses of all these elements constitutes alarge mass, which, in view of an extremely inefficient heat transfergradient, results in a heating rate that is exceptionally slow andineflicient.

In the present invention, shorting is precluded even though the tantalumemployed are disposed in much closer proximity to each other than hasheretofore been possible. Thus an increased concentration of heaterswithin the work area vastly increases efiiciency and permits the brazingtemperature to be driven much higher during any given time period.

With the brazing temperature substantially above the point at whichrecrystallization begins, it is obvious that the time required toachieve the brazing temperature time within the recrystallizationzone-is extremely critical, and that prior furnaces having inherentlyslow and inelficient heating rates destroy the intrinsic strength andheat resistant qualities of the material upon which they were designedto operate.

It has been demonstrated that time within the recrystallization zone isextremely detrimental to various materials and that great inefiiciencesresult as an effect of the slow heating rate. However, a slow coolingrate is obviously equally as detrimental. Therefore, .any process mustbe evaluated by the total time it requires within this zone, whichdetermines the extent of joint degradation that is ultimately sustainedby the materials involved. As hereinabove mentioned, all interiorcomponents of the conventional furnace and the workpiece must besaturated with heat to attain the metal bond. This result-s insubstantial residual heat at the termination heating cycle. Thisresidual heat dictates the necessity for an extremely long coolingcycle, thus further lengthening the time that the object material issustained above its recrystallization point.

Another deleterious characteristic of the prior devices and theirresultant methods resides in the high initial expenses of fabricationand subsequent excessive cost of operation due to inherent inefficiency.

Another apparatus which is similar in principle, although totallydifferentiated in result, to that of the present invention is disclosedin the T. A. Herbert patent, No. 3,033,973. This apparatus is of theelectric blanket type and permits better dwell times than thosedelineated above. Even so, this device is limited by the type of heaterand electrical and thermal insulators employed therein to a maximumoperational temperature of 1700 F. This limitation precludes itsemployment in processing of the exotic and refractory metals madepossible by the present invention.

The present invention resides primarily in the novel utilization of athermally stable electrical insulator which exhibits an extremely highrate of heat transfer through conduction while being, at the same time,metallurgically compatible with the refractory metal heaters employedtherewith. This extremely high rate of heat transfer and metallurgicalcompatibility permits the attainment of extremely high temperatures withvery brief heating-cooling cycles. This ability permits the requisitesuper alloy or refractory metal brazing temperature to be attained, butprevents temperature maintenance above the recrystallization temperaturefor time intervals which cause substantial degradation of the joint.That is, the present invention permits attainment, for the first time,of proper tem perature-dwell time relationships permissive of achievingmetallurgically and structurally satisfactory bonds between super alloyand refractory metals in an economical and relatively simple devicecapable of general utilization by any person having skill in the art.

Specifically, the device of the present invention employs a thermallystable ceramo-metallic or refractory oxide type electrical insulator incombination with refractory metal heaters, the insulation beinginterposed between parallel, juxtapositioned ribbon heaters and theobject workpiece. Such positioning permits the optimum current,heating-time cycle, and efficiency, but prevents direct electricalshorts or arcing.

In the preferred embodiment of the present invention the heaters aredisposed upon either side of the workpiece having only insulatorstherebetween, thus preventing electrical shorts as hereinabove stated.This configuration allows the workpiece to be substantially in physicalcontact with the heating media. Obviously, this method of heatingpossesses an intrinsically high rate of thermal conduction andefficiency, thereby promoting very rapid heating rates. Further, sincethe heaters are adjacent to the workpiece, and the heat generated isconfined by suitable means to the area occupied by the workpiece, onlythe areas immediately adjacent to the workpiece are heated, therebysubstantially reducing the mass that is heated, as well as substantiallyreducing residual heat. The combination of contact heating, high powerinputs, and reduction of the mass that is to be heated results in asignificant reduction of the dwell time within the recrystallizationzone, and allows greater temperatures to be reached much more rapidly.These facts, in combination with the reduction of residual heat and aflow argon atmosphere further result in a very abrupt cooling cycle.

Ceramo-metallic insulators are also employed to preclude deleteriouschemical reactions experienced when the refractory metal heaters contactthe thermal insulating means. This chemical reaction, if allowed,results in the deterioration of the heaters and subsequently in heaterfailure. Further, such insulators function as a heat distributing means,which is important because the heat generated by the heaters islocalized due to their strip configuration. This distribution iseffected due to the positioning of the electrical insulators between thework and heating media, thereby uniformly conducting the generated heatover the entire area to be joined.

The novel combination of elements of the device of the present inventionpermits its efiicient operation up to a maximum temperature of between3900 F. to 4700 F. dependent upon the type of insulator used.

Ceramometallic or refractory oxide insulators according to the presentinvention are comprised of a refractory metal sheet, which in thepreferred embodiment is molybdenum and an oxide coating. The metal sheetis slightly larger than the workpiece to permit beveling at the edges,thus precluding abrasions in the oxide coating and avoiding sharp edgesat any point of contact with the heaters. The refractory oxide insulatorcoating may be applied in either of two methods.

One method is to first paint a thin coat of levigated alumina on therefractory sheet in a slurry of 2 percent poly-vinyl-alcohol and 5percent sodium silicate and subsequently bake the encapsulated sheet at250 F. until dry. The sodium silicate improves adherence of the oxide tothe sheets. A second identical coat is then applied degrees to thestrokes of the first coat. The second coat is followed by a baking at900 F. for one hour to drive off the major portion of thepoly-vinyl-alcohol. The alumina alcohol-sodium silicate mixture shouldpreferably be about the consistency of oil base paint. It should benoted that the insulators are not limited to molybdenum sheets oraluminum oxide for any refractory metal and that most oxides willsuffice, however, beryllium oxide, magnesium oxide and thorium oxidehave been found to be the most useful. A second method of applicationresides in flame spraying the oxide to the supporting molybdenum sheetand has been found to be extremely satisfactory. Obviously, the methodemployed is a matter of choice.

The salient object of the present invention is therefore to provide anapparatus operable within an extended range of temperatures up toapproximately 4700 F. and capable of favorable dwell times, and aprocess to effect both brazing and diffusion joining of refractorymetals and super alloys, thereby permitting fabrication of hightemperature structural components, such as low density cellular core andhoneycomb sandwich panel, while simultaneously precluding detrimentalchanges within the crystalline structure thereof.

It is another object of the invention to provide an electrical insulatorpossessing a very high heat transfer rate which comprises a refractorymetal sheet encapsulated within a refractory oxide coating. Such coatingpermits contact heating of the workpiece thereby reducing the timerequired for the heating and cooling cycles.

It is still another object of the present invention to provide anapparatus of the class described which employs high temperaturerefractory metal resistance heaters.

It is yet another object to reduce high tooling and fabrication costs ofpresently available brazing apparatus.

Other and further features and object of this invention will becomeapparent to those skilled in the art in light of the followingspecification and drawings wherein:

FIGURE 1 is an elevational View, partly in section, of the device of thepresent invention in the preferred form;

FIGURE 2 is a pictorial, exploded view of the heaters and separators ofthe present invention revealing their typical relation to each other.

The preferred embodiment of the present invention, as shown generally inFIGURE 1, comprises a plurality of parallel juxtapositioned ribbonresistance heaters (best shown in detail in FIGURE 2), which heatersform heater banks 12 and 14, heater banks 12 and 14 being employed ondiametrically opposed sides of a workpiece 16. Heater banks 12 and 14are evenly spaced over the area to be processed and comprise a pluralityof pure molybdenum ribbons, which in one preferred embodiment are of aninch wide, .002 of an inch thick and 14 inches long. Spacing of ribbonelements 10 is maintained by doublers 18 and 20 positioned across theribbons at their longitudinal extremities. It has been found that spotwelding .005 inch columbium strips is satisfactory. Contact strips 22,24, and 26, are positioned on the ends of heater banks 12 and 14respectively to transmit electrical power to the individual heaterstrips 10. This power transmission is accomplished by connection to twocopper bus bars 30 and 32, which are subsequently attached to a DC powersupply shown schematically in FIGURE 1.

Electrical insulation between heater banks 12 and 14 and workpiece 16,as well as hereinafter described insulating bricks, is effected by fourthin ceramo-metallic insulators 34, 36 and 38, 40 illustrated inFIGURE 1. Insulating bricks 42, which in one preferred configuration are8 inches in length, 8 inches in width and 2 /2 inches in height, areemployed to localize heat. Bricks 42 are bound together into slabs 44 byan adjustable strap 46 which may be of A-286 alloy. Bricks 42 arepositioned as illustrated in FIGURE 1 above and below workpiece 16, andits associated insulators 34, 36, heater banks 12 and 14 and insulators38, 40 respectively. Prior to the placement of edge bricks 48 and 50, astrap 52, which may be of titanium or the like, ispositioned around theperiphery of the workpiece to further prevent heat loss and to burn anyoxygen present. Edge bricks 48 and 50 in one preferred embodiment 8inches in length, /2 of an inch in height and of an inch in width, andare placed around the edges of workpiece 16 adjacent to strap 52. Asmall aperture (not shown) may be desirable for optically measuring theinterior temperatures at the workpiece.

All of the above described components are positioned on a suitable base54 having spacers 56 thereon in order to position the workpiece inproper relation to the heaters.

A suitable argon inlet 58 is mounted within base 54. Base 54 issubsequently provided with an air tight cover 60 thereby effectivelyforming a retort. Weights 62, as shown in FIGURE 1, may be employed toobtain any required pressure on workpiece 16 and are positioned uponupper insulating bricks 42. Other means of applying pressure may beemployed such as differential pressure and/ or any suitable mechanical,hydraulic or pneumatic means.

The process of the present invention is initiated with a first purge ofretort 60. It has been found to be desirable to employ a three cyclepurge. Each cycle consists of vacuum pumping to less than 200 micronsabsolute pressure, followed by back filling with argon to ambientpressure. After purging is complete, the argon flow is adjusted to asteady 20 ft.*/ hr.

After purging the heating cycle is accomplished in two steps. The firststep involves removal of an acrylic resin matrix in the powdered brazealloy and may be omitted if such a material is not employed. Whereemployed, this step consists of slowly increasing the temperature toapproximately 1600 F. and holding at this temperature until all signs ofsmoke disappear. The second step resides primarily in rapidly increasingthe power input to achieve a rate of heating of approximately 200 F. perminute. This rate is maintained until the recrystallization zone isreached. At this time, the input is suddenly increased, resulting in anextremely rapid increase to brazing temperature. This increase normallyrequires only thirty seconds. Power is then terminated in order tominimize dwell time above the recrystallization temperature of thematerial to be brazed. The cooling rate is then recorded in one minuteincrements down to about 1100 F. In this process, employing thedisclosed apparatus, the refractory materials being joined are withinthe recrystallization zone less than one minute, thus precludingdetrimental recrystallization and grain growth.

For example, in a preferred embodiment wherein the material to be brazedis columbium FS-82 alloy, the process employed is as follows: Thematerial was set up in accordance with the prior description. The retortwas purged three times as explained above. The temperature was raisedgradually to 1600 F. and held until smoking disappeared. The power inputwas rapidly increased to 7 kilowatts, resulting in a rate of heating ofabout 200 F. per minute, which rate was maintained until a temperatureof 2300 F. was achieved. The input was then increased suddenly to 8kilowatts, resulting in a very rapid increase to the brazingtemperature. This increase required only 30 seconds to complete. Powerwas then terminated, permitting a very rapid temperature falloif tobelow the recrystallization zone. The material was within this zone forless than 60 seconds total.

This is best illustrated in the following sandwich panel columncompressive tests which verify that the present invention effectsjoining without significant recrystallization and grain growth withtheir corresponding loss of mechanical properties.

Room Temp. Column Oompresslve Strength After Brazing (p.s.i.)

Room Temp. Ultimate Tesile Strength of Skin Materials Before Brazing(p.s.i.)

Panel Material Run No. Columbian 5 Alloy Industry wide evaluations ofbrazed sandwich panels indicate that in the optimum condition, columncompressive strengths should equal or slightly exceed the ultimatetensile strength of the skin material before brazing, thus indicatingretention of full tensile properties in substantial absence ofrecrystallization and 'grain growth. As is obvious from the above, thiscondition is fully satisfied in materials brazed by the process of theinvention.

As thus described above in detail, the present invention for the firsttime provides a practical process and apparatus operative to braze ordiffusion join refractory or super alloy materials without loss ofdesirable structural characteristics through intrinsic recrystallizationand grain growth.

This is made possible by employment of a heat source comprisingrefractory metal strip resistance heaters. These heaters, due to theirconstruction and ceramo-metallic insulation, may be positioned in veryclose proximity both to each other and to the workpiece being joined.This in turn effects a rapid heating cycle through extremely high heattransfer efficiency. In addition, such positioning substantiallyprecludes extraneous heating of supporting and insulating components andthus deters any buildup of residual heat. This effectively decreases thecooling cycle, which in combination with the related rapid heating ratesachievable by the invention appreciably reduces dwell time within therecrystallization and grain growth zone.

What is claimed is:

1. A process for joining refractory metals and super alloys withoutsubstantially deleterious recrystallization therein comprising:

(A) selecting the material to be joined;

(B) positioning said material in a closed environment;

and in contact with a refractory metal electrical resistance heatermeans and applying a braze alloy to the surfaces of said material to bejoined;

(C) heating said material slowly by controlled graduated steps to apoint just below that at which recrystallization begins;

(D) causing the temperature of said material to rapidly increase to thebrazing temperature of said selected material;

(E) terminating the temperature producing function and effecting a rapidtemperature decrease in said selected material to a point below therecrystallization zone for said selected material so that the materialis above the point at which recrystallization begins for no more thansixty seconds.

2. The process as defined by claim 1 wherein the braze alloy is apowdered alloy in an acrylic resin binder and said step of heating bycontrolled graduated steps includes:

removing said acrylic resin binder by heating said material tosubstantially 1600 F. and holding until smoke disappears.

3. A process for brazing refractory and super alloy materials resultantin a maximum of about 5% recrystal- 70 lization and thus precludingsignificant loss of mechanical properties comprising:

(A) selecting the materials to be joined and applying a braze alloythereto in the areas to be joined;

(B) positioning the materials between and in contact with refractorymetal insulative separator means to provide substantially uniform,direct heating of said selected material by a heater;

(C) providing a refractory metal, rapid electrical resistance heatermeans in contact with the separator means;

(D) positioning second separator means on the heater means to provideelectrical insulation;

(E) thermally insulating the materials, heater means and separators;

(F) purging the environment around said selected materials;

(G) slowly raising the temperature of said materials to a temperature ofapproximately 1600 F. by controllably activating the heater means;

(H) rapidly raising the temperature of said materials at about 200 F.per minute to a first point just below that at which recrystallizationof said selected metals begins;

(I) raising the temperature from said first point to the materialsbrazing point in about thirty seconds;

(J) terminating the temperature imparting operation to cause a rapidtemperature drop in about thirty seconds to a second point below therecrystallization zone to thus minimize dwell time.

References Cited UNITED STATES PATENTS 3,047,710 7/1962 Rowe 219-853,053,969 9/1962 Kerr et a1. 219-85 3,067,508 12/1962 Kinelski 29-487 103,167,634 1/1965 Kreiter et al. 219 11s.2 x 3,173,813 3/1965 Dewey et a1148-127 X 3,276,113 10/1966 Metcalfe 29-487 OTHER REFERENCES 15Molybdenum, Production, Properties and Applicants, by G. L. Miller,Metal Industry, Nov. 18, 1949, pages 439-441.

RICHARD M. WOOD, Primary Examiner. 20 B. A. STEIN, Assistant Examiner.

1. A PROCESS FOR JOINING REFRACTORY METALS AND SUPER ALLOYS WITHOUTSUBSTANTIALLY DELETERIOUS RECRYSTALLIZATION THEREIN COMPRISING: (A)SELECTING THE MATERIAL TO BE JOINED; (B) POSITIONING SAID MATERIAL IN ACLOSED ENVIRONMENT; AND IN CONTACT WITH A REFRACTORY METAL ELECTRICALRESISTANCE HEATER MEANS AND APPLYING A BRAZE ALLOY TO THE SURFACES OFSAID MATERIAL TO BE JOINED; (C) HEATING SAID MATERIAL SLOWLY BYCONTROLLED GRADUATED STEPS TO A POINT JUST BELOW THAT AT WHICHRECRYSTALLIZATION BEGINS; (D) CAUSING THE TEMPERATURE OF SAID MATERIALTO RAPIDLY INCREASE TO THE BRAZING TEMPERATURE OF SAID SELECTEDMATERIAL; (E) TERMINATING THE TEMPERATURE PRODUCING FUNCTION ANDEFFECTING A RAPID TEMPERATURE DECREASE IN SAID SELECTED MATERIAL TO APOINT BELOW THE RECRYSTALLIZATION ZONE FOR SAID SELECTED MATERIAL SOTHAT THE MATERIAL IS ABOVE THE POINT AT WHICH RECRYSTALLIZATION BEGINSFOR NO MORE THAN SIXTY SECONDS.